Process for applying protective metallic coatings



PROCESS FOR APPLYING PROTECTIVE METALLIC COATINGS Dwight G. Moore, Washington, D. C., assignor to the United States of America as represented by the Secretary of the Air Force Application November 9, 1954, Serial No. 467,901 7 Claims. (Cl. 117129) (Granted under Title 35, U. S. Code (1952), see. 266) The invention described herein may be manufactured and used by or for the United States Government for governmental purposes without payment to me of any royalty thereon.

This invention relates to a new and an improved protectively coated metal article, to a new and improved surface coat for metal articles, and to a process by which a protective coat is adhered to a metal substrate.

Aircraft power plants, articles made of steel, cast iron alloys and other metals, have been protected in the past from corrosion, weathering and other deteriorating agents by protective surface coatings applied to the articles by electro-deposition, hot dipping, flame spraying and derivative processes. In the flame spraying practice, a coating metal in the molten state has been sprayed on the clean surface of a base or substrate metal.

Serious disadvantages attend the flame spraying method of applying a protective material to a metal surface. The spraying of hot molten metal is not an easy opera- -tion and the method is not inexpensive. Preparatory to the spraying operation, both the metal to be sprayed and the base metal must be maintained clean and free from oxides, scale and the like, in order that satisfactory adhesion between the base metal and the molten metal may be accomplished. It is difficult or it may be impossible to apply a molten metal spray to some types of surfaces which are not readily accessible to the spray, such as the inside surfaces of cylindrical sections of small diameter, small pipes and small apertures in complex designs and intricate patterns and the like. Products made by the flame spraying method commonly require the machining or the grinding of the surface coating after the work is cooled. The flame spraying of metal on a substrate or base metal is objectionably characterized by excessively thick coats which lack uniform depth. Protective metal coats of this type are not ductile and objectionably fracture when subjected to bending stresses.

In the field of previously issued patents using ceramic coating techniques, steel has been protected by the application of a paste containing two alloys and a flux. The surface coat is then dried and fired to fusion after which the work is cooled and the surface slag is brushed away from the surface coat. Illustrative alloys used in this process are an 8020 mix of nickel and ferrosilicon, a mixture of ferrochromium, ferrosilicon, boron, etc. to which is added an illustrative flux containing boron compounds, silicon, etc. Another patent discloses a ceramic coating for iron, steel and alloys, wherein a metal surface is thoroughly cleaned and a coating in the form of a frit in a mill batch is applied as a slip. The frit may contain quartz, feldspar, borax, one-half percent nickel oxide, etc. The mill batch consists of a frit, diaspore, water and either borax or a mixture of flint clay and soda ash. The coatings are applied and bonded to the metalby again following ceramic coating procedure. Another patent using a ceramic coating procedure discloses oxides of nickel, chromium, etc. a flux of lead borosilicateglass, borax, etc. and water and the mixture 2,857,292 Patented Oct. 21, 1958 2 ground to a slip. The slip is applied to a clean iron or steel base metal from which all oxides and scale have been removed. The applied coat is dried and is then fired at 1550 F. from 15 to 45 minutes, preferably in a reducing atmosphere after which the surface is buffed. Another patent using ceramic coating procedure for applying a protective coat to the surface of base metals discloses an enamel ground coat consisting of two parts by weight of nickel oxide to each part of chrome ore with silica added. The enamel ground coat slip is applied to cast iron, steel, an alloy or the like, dried and fused at a temperature from 1600 to 1900 F. for a time from one-half hour to one hour. In this procedure, the iron, steel or alloy mill scale is not necessarily removed previous to the ceramic coating application to the piece, for the reason that the enameled ground coat when fired fuses through to the metallic base or to the substrate.

The present invention provides in the field of metallic articles, an article having a surface which is protected by either a metal coat, a cermet coat, or a coat consisting of a ceramic distributed within a metal or a cermet lattice. The protective coat of the improved article is firmly and intimately bonded to the substrate metal. The protective coat of the improved article has the composition of an eutectic of the metals nickel, boron and chromium, either alone or with a cermet, which is a ceramic metal mixture, or with a glass phase entrapped within a lattice made of the eutectic metal or of the cermet. A eutectic Y pressure. The base metal may be of iron, steel, an alloy or other metal, degreased but from which the removal of the scale and any oxides present from either the base metal or the metals in the eutectic material is unnecessary. The present invention is adapted to provide, as a protective coat on the base metal, an all metal layer, or a welded matrix of metal or of a cermet, with a ceramic material filling the interstices of the matrix.

One object of the present invention is to provide an economically advantageous and an improved metal article having a surface coat which is highly resistant to corrosion, weathering, abrasion, thermal shock, and the like.

Another object is to provide for metal articles, an easily applied protective surface coat which is of substantially uniform thickness, which is firmly bonded to the base metal, and which has a highdegree of ductility such that the cracking of the protective coat under bending stresses and the like, is minimized.

Another object is to provide a new and improved metal article protective coat consisting of a layer of a metal or .of a cermet or of a lattice of one or more of these materials with a ceramic material filling the interstices in the lattice.

A further object is to improve the utility in high temperature service of low strategic metals such as low alloy steel, ingot iron, and the like.

A further object is to provide an improved ceramic coating technique which is advantageous for large scale production in applying protective surface coats to metallic articles.

A further object is to provide a method following ceramic coating techniques by applying a water suspension of a coating metal and a flux to a base metal at roomtemperature and then bonding the coating metal to the base metal by a firing operation to produce a finished article requiring no further operations for its completion.

Another object is to provide an improved ceramic coating technique for the application of a protective surface coat to metal articles having fine and intricate pata terns inclusive of small apertures without sealing over or obstructing the apertures.

In practicing the process and the method by which thecoating of aircraft'power plants or other metal. articles contemplated hereby are produced, preferably a chromium-boron nick'el cermetmixture of substantially eutectic proportions is mixed with a-frit, clay and water. to: make a slip; The article to be coated then has the resultant watersusp'ension slip applied thereto by a dipping, a. brushing or a spraying operation. The water suspension coat on the article is then air-dried, and the article so coatedis fired ata surface coating, fluxing temperature within. or near a range of from 1850 to 2000 F. for a time adequate either to melt or to begin to meltboth the chromium-boron-nickel eutectic material and the flux without objectionably softening or meltingthesubstrate andto bring the work up to the furnace temperature. Uponthe completion of the fir.- ing, the. work is permitted to coolin air without unusual attention.

In the accompanying drawings are shown sketches illustratingrthe microstructure of a chromium-boron-nickel cermet coating which, in Fig. 1 is fired at a lower temperature of 1900 F; and which, in Fig. 2 is fired at a higher temperature of 2000 F. In the two sketches A represents a metal or an alloy substrate, B is a chro miurn-boron-nickel phase welded to the substrate at D and C. indicates a glass phase. The artistic representations. shown in Figs. 1 and 2 in the accompanying drawing are presented with photographic accuracy at page 380 in The American Ceramic Society Bulletin, volume 34, No. 11, published November 15, 1955.

A preferred cermet eutectic material applied to alow carbon base metal and suitable for high temperature service, has-the following approximate proportional composition of ingredients expressed in weight percentages.

Ingredient: Weight percent Ni 65 M75 Cr 13 to 20 B 3 to 5 Fe, Si, C max.

A preferred glass or ceramic flux, among several experimentally successful compositions, is identified herein in the year 1954 as National Bureau of Standards frit No. 332 and consists of the following computed weight percentages of-ceramic oxides:

The above composition is derived from a representative batch of the following proportional parts by weight:

Ingredient: Parts by weight Barium carbonate 56.65 Flint (SiO 37.50 Boric acid- 11.50 Zinc oxide 5.00 Calcium carbonate 6.25 Zirconium oxide 2.50 Hydrated alumina 1.52

Parts total 120.92

The above materials experimentally were pulverized'to pass a U. S. Standard 40 mesh sieve.

A preferred coating mixture consists by weight of 90 parts metal. Cit-.B-Ni eutectic material, 10' parts frit 4. (glass), 5 parts clay (kaolin), and 40 parts water, making a total mix of 145 parts. The clay preferably is an enameling clay, specifically designed for use in porcelain enamel ground coats.

The physical state of the ingredients prior to their being mixed is powdered, with the cermet eutectic material and the ceramic frit batch illustratively pulverized to pass a sieve of about mesh to the inch. The mixing of the ingredients may be accomplished as a thoroughly and a uniformly distributed water suspension slip. If preferred, all of the ingredients may be mixed simultaneously by being ground in a ball mill to a desired degree of fineness. For coats a few thousandths of an inch in depth, the ingredients are ground to a greater degree of fineness than for thicker coats.

The application of the resultant slip as a coat of substantially uniform thickness may be accomplished by a desired procedure, such as by dipping, spraying, brushing or the like, on: the degreased metal substrate to be coated. The water content of the-applied slip is then: removed by evaporation.

The coated work. preferably is fired in air or in an atmosphere low in oxygen,-at a temperature sufficiently high and for a time sufiiciently long to melt or to begin to melt both the Cr-B-Ni eutectic material and the frit without softening the substrate, after the work has arrived at the furnace temperature. range of about 1850 to 2000 F. and a firing time of four minutes: were found to. be adequate experimentally for an 18 gauge base metal. Upon the completion of the firing operation the work is permitted to cool normally in air to a finished. product.

In the firing operation, the glass frit servesas' a'- fluxby-dissolvingtheoxidespresent on the surfaces of both the base metal and the coating metal and permitting the eutectic to weld or to braze itself with the base metal surface. This scavenging action permits a mutual wetting action between the eutectic metal and the surface of the substrate or'base'metal and-upon'their solidifica-' tion or. freezing permits a-mutually strong adhesion to occur therebetween.

Microphotographs in- The American Ceramic Society Bulletin of experimental sections fired at 1900 F. show in the layer material a definite line ofdemarcation between the substrate and the" eutectic material and a' clearly defined distributed segregation of'the glass frit within and upon the eutectic. Fired at 1950 F. the line of demarcation between the substrate'and the eutectic is noticeably less clearly defined; and the occlusions of" glass frit in the eutectic are markedly decreased with clearly defined interfaces" and with definite segregation in layer thickness of the glass frit on top of the eutectic and remote from the base metal. Fired at 2000 F. a definite intermingling or almost mutual fluidity or alloying is apparent at the junction of the eutectic and the base metal, insuring a strong bond therebetween when solidified; and the eutectic'is substantially free from frit occlusions with definite segregation in layers of the eutectic and the frit with a frayed interface therebetween.

The substrate in all three samples displays materially less crystalline internal structure than prior to the pro.- longed heat treatment. The specimen initially fired at 1950 F. continues to display within the. eutectic ap preciably sized occlusions of frit but the occlusions are spaced somewhat away from the interface between the eutectic and the base metal and the'eutectic is'rnore free from frit than is the specimen initially fired at 1900 F.

The specimen initially fired at 2000 F. when given" 800' hours heat treatment at 1500 F., displays about half-way down in theeutectic layer, a layer of. small occlusions of frit; indicating the localized coalescence offinely distributed frit as a result of the prolonged" The interface'between' the base metal A firing temperature" The above experimental results indicate that under the above described conditions, after having served its purpose as a flux, the glass phase of the coating bleeds out to the surface. After being cooled down to room temperature the outer glass layer flakes away from the remainder of the coating, leaving as a surface cover a clean, continuous and uninterrupted layer of protective noncorroding and abrasion resisting metal of more nearly uniform thickness than has been possible of production heretofore.

By a desired selection of ingredients, a protective metallic layer having desired properties of hardness, flexibility, abrasion resistance, corrosion resistance, oxidation resistance and the like, may be permanently welded or brazed to a substrate by the application of eutectic metal on a base metal, using the ceramic coating technique which is disclosed herein.

The glass flux hardened on top of the metal coating is characterized by being a fragile, poorly adherent layer. The glass flux layer may be removed by mechanical scraping, a light sand blast, or the like.

The glass flux may be caused to remain embedded within the eutectic, where such structure is desired, using the same composition and firing at a lower furnace temperature, such as between 1850 and 1900 F. The resulting structure is characterized by the glass phase remaining occluded within and remaining as an integral part of the protective coating on the base metal.

The thickness of protective coats applied to a base metal by the process disclosed herein has ranged upwardly experimentally from a coat 0.001 inch thick through coats up to 0.030 inch thick. The thickness or the depth of protective coats applied to base metals as disclosed herein, may be applied in multiples, particularly when applying coats of thicknesses exceeding a coating 0.01 inch in depth, to any desired thickness, within the scope of the present invention.

Adequate fiuxing action, absorbing the oxides, scale, etc. from both the substrate and from the metals in the eutectic, is provided by any of a number of frits which were used experimentally and known to the trade.

The proportions of the ingredients in thecoating mixture may be varied over a wide range. The range of the frit, illustratively, may be varied from 5 to 50 weight percent. As the proportion of frit in the coatings is varied, different results are obtained, as to be expected, such as the above mentioned coat wherein the glass flux is retained embedded within the eutectic. This type of coat is desirable for the protection of certain metals which are to be exposed to high temperatures. Glass flux retaining eutectic coats may be obtained by using different proportions of frit, by the addition of refractory oxides and by using different firing temperatures.

Factors determining whether the protective coating on the substrate shall consist entirely of metal or shall consist of a network of metal in which the interstices are filled with ceramic material have been worked out experimentally.

When a relatively low proportion of frit-to-metal is used in the coating mixture, the frit migrates to the surface of the eutectic at a relatively low firing temperature, leaving a layer of solid metal bonded to the substrate. In the above mentioned preferred coating composition by weight of 90 parts metal Cr-B-Ni eutectic to 10 parts frit, it has been found experimentally that when fired at about 1900 F. the frit separates substantially completely from the metal layer. In a composition of 70 parts metal and parts frit, the firing temperature must be' raised to about l950 F. before the substantially complete separation of the frit from the metal layer occurs. In a composition by weight of 50 parts metal and 50 parts frit, the firing temperature must be raised to about 2l00 F. before the frit substantially completely separates from the metal layer bonded to the substrate.

A protective layer of ceramic filled metal lattice at tached to the substrate is accomplished for improved performance at higher'temperatures by the use of a relatively low metal to frit ratio, fired at a temperature at which the metal phase liquefies but at which the temperature isnot high enough to cause the frit to segregate itself upon the surface of the metal coat. Such a mix comprises, for example, by weight: 50 parts metal, 50 parts frit, 5% clay and 40 parts water, for a weight total of 145 parts, with the mix fired at 1950 F. For some applications a protective coat of this composition has better resistance to corrosion than has an all metal coat.

Another ceramic loaded metal lattice coat is accomplished within the scope of the present invention by the addition to the metal-frit-clay and water slip, illustratively, of the preferred metal to 10 frit composition, of a refractory oxide such as A1 0 or Cr 0 in about 5 parts by weight and fired at l950 F. The admixture to the slip of the refractory oxide increases the viscosity of the frit or glass phase and retards its migration to the surface of the metal phase.

Base metals or substrates experimentally coated successfully in practicing the invention which is disclosed herein, have included low carbon steel, molybdenum and numerous nickel-chromium alloys and nickel-chromiumiron alloys. A steel alloy containing 2 weight percent chormium was successfully coated and laboratory tests indicate that the coated alloy should be suitable for its use at temperatures up to 1200 F. in heat exchangers, hot piping in oil refineries, in cracking stills and the like.

The nickel-boron-chromium eutectic metal, frit, clay and water composition, which is disclosed herein, has been used experimentally to coat the above base metals or substrates both with all metal coats and with coats of ceramic or glass on metal.

One modification in batch composition and for purposes of designation referred to herein as ceramic-metal coating M-55, experimentally consisted in parts by weight of the ingredients: Cr-B-Ni metal powder 50 parts; frit NBS332 as mesh powder 40 parts; fused alumina of 90 mesh 10 parts; enameling clay 5 parts; and water 25 parts for a total of parts.

In the milling of the ingredients of this ceramic-metal coating M-SS, for an illustrative 260 gram batch of eutectic metal, alumina and frit, the mill charge consisted of 100 grams of Cr-B-Ni eutectic metal powder, 20 grams of alumina, 80 grams of frit NBS332, 10 grams of clay, and 50 grams of water. This batch was milled in a one quart jar for one hour.

The milled slip so prepared, was removed from the jar mill and water was added to double its volume. The thinned slip was stirred well and then was allowed to settle for several hours. After settling, the supernatant water was poured from the top of the slip, the slip was stirred and additional water was added until a dipping thickness of the slip when dry was about in the range of from 0.012 to 0.015 inch. The purpose of the dilution with water and its subsequent decantation is for removing entrapped air bubbles.

The slip so conditioned was applied by dipping to a metal substrate, the specimen was air dried and then it was fired in air at a range of from 1900 to 1950 F. for a sufficient time to permit the specimen to be at furnace temperature for at least two minutes. In the event a denser and smoother surface, or a sealing coat, is desired on the resultant article, it may be obtained by further ceramic coating applied in a similar manner.

Further heating of the resultant product, such illustratively as for 90 hours at 1800 F. in an air atmosphere, does not weaken the bond between the Cr-B-Ni metal particles and the substrate but it does promote some flaking of the outermost layers of the coating. This condition can be corrected somewhat by imparting a more continuous metallic network to the coating by increasing the proportionate amount of Cr-B-Ni eutectic metal powder in the mix.

It is to be understood that the protectively coated article, the composition of the coating, and the particular ceramic coating process, which are disclosed herein have been submitted for the purposes of illustrating and explaining a successfully operative embodiment of the present invention and that similarly functioning modifications therein may be made without departing from the scope of the present invention.

What I claim is:

1. A metal article consisting of a substrate protected against corrosion, abrasion and weathering by a surface coat brazed to the substrate and the surface coat consisting of a cermet lattice entrapping a ceramic within its interstices with the cermet consisting gravimetrically of about 90% of the alloy of between 65 and 75 percent nickel, between 13 and 20 percent chromium and between 3 and 5 percent boron and about 10% of a frit which consists of quartz, feldspar, borax and nickel oxide and wherein the alloy contacts the substrate in a continuous protective coat and the frit contacts the alloy in a continuous protective coat.

2. A slip composition, consisting of by weight about 90 parts of metal Cr-B-Ni eutectic, about 10 parts of frit made of quartz, feldspar, borax and nickel oxide, about 5 parts of clay, and about 40 parts water.

3. A slip composition, consisting of by weight about 90 parts of metal Cr-B-Ni eutectic, about 10 parts of frit consisting of quartz, feldspar, borax and nickel oxide, about 5 parts of A1 about 5 parts clay, and about 40 parts water.

4. A slip composition, consisting of by weight about 90 parts of metal Cr-B-Ni eutectic, about parts of frit made of quartz, feldspar, borax and nickel oxide, about 5 parts of Cr O about 5 parts'clay, and about 40'parts water.

5. A slip for application to a base metal, the slip consisting of a composition by weight from about 50 to 90 parts metal Cr-B-Ni eutectic, of from about'10 to50 parts of frit of the glass composition quartz, feldspar, borax and nickel oxide, about 5 parts of clay, and of from about 25 to 40 parts water.

6. The process for applying a protective and an adherent coat to the surface of a base metal; which comprises providing a mixture of the powdered metals; chromium, boron, and nickel in substantially eutectic proportions, powdered glass frit of quartz, feldspar, borax and nickel oxide, and clay; adding water to the mixture about to a slip composition; grinding the mixture to a particle fineness capable of passing a screen of 100 mesh to the inch in a substantially uniform distribution in a slip; applying the slip in substantially uniform depth upon the surface of the base metal; drying the slip on the base metal; and firing the dried slip on the base metal within or near a temperature range of from 1850 to 2100 F. to the fusion of the dried slip and its even distribution on the base metal as a protective coat of substantially uniform depth strongly and permanently adherent to the surface of the base metal.

7. A method for applying a protective coat to a metal which consists in providing a mixture of the metals: chromium, boron and nickel in substantially eutectic proportions and in powdered physical state, with a powdered glass frit consisting of quartz, feldspar, borax and nickel oxide and a clay; adding water to the mixture to provide a water suspension; applying the water suspension to a substrate as a coating thereon; removing the water from the substrate coating; and firing the coating on the substrate in a reducing atmosphere at about 2000 F. in adhering the coating to the substrate as a protective covering therefor.

References Cited in the file of this patent UNITED STATES PATENTS 2,570,649 Davidofi Oct. 9, 1951 2,585,430 Boegehold Feb. 12, 1952 2,694,647 Cole c- Nov. 16, 1954 2,774,681 Huppert et a1. Dec. 18, 1956 2,775,531 Montgomery et al. Dec. 25, 1956 FOREIGN PATENTS 501,069 Schwarzkopf (Canada) Mar. 30, 1954 OTHER REFERENCES Metal Finishing, vol. #10 October 1952, p. 61. 

6. THE PROCESS FOR APPLYING A PROTECTIVE AND AN ADHERENT COAT TO THE SURFACE OF A BASE METAL; WHICH COMPRISES PROVIDING A MIXTURE OF THE POWDERED METALS; CHROMIUM, BORON, AND NICKEL IN SUBSTANTIALLY EUTECTIC PROPORTIONS, POWDERED GLASS FRIT OF QUARTZ, FELDSPAR, BORAX AND NICKEL OXIDE, AND CLAY; ADDING WATER TO THE MIXTURE ABOUT TO A SLIP COMPOSITION; GRINDING THE MIXTURE TO A PARTICLE FINENESS CAPABLE OF PASSING A SCREEN OF 100 MESH TO THE INCH IN A SUBSTANTIALLY UNIFORM DISTRIBUTION IN A SLIP: APPLYING THE SLIP IN SUBSTANTIALLY UNIFORM DEPTH UPON THE SURFACE OF THE BASE METAL; DRYINUG THE SLIP ON THE BASE METAL; AND FIRING THE DRIED SLIP ON THE BASE METAL WITHIN OR NEAR A TEMPERATURE RANGE OF FROM 1850 TO 2100*F. TO THE FUSION OF THE DRIED SLIP AND ITS EVEN DISTRIBUTION ON THE BASE METAL AS A PROTECTIVE COAT OF SUBSTANTIALLY UNIFORM DEPTH STRONGLY AND PERMANETLY ADHERENT TO THE SURFACE OF THE BASE METAL. 